1. Field of the Invention
The invention relates to a method of making a semiconductor crystal by using an ELO (epitaxial lateral overgrowth) mask of amorphous material formed on a crystal growth surface to offer lateral growth of crystal. Also, the invention relates to a semiconductor light emitting element made by using the method.
2. Description of the Related Art
Several methods of reducing a dislocation or void in the crystal to make a good semiconductor crystal by using an ELO mask are known (e.g., prior arts 1 to 4 below).
Japanese patent application laid-open No. 2000-21789 (prior art 1) discloses that the number of voids or cracks can be relatively reduced by using an ELO mask formed on an underlying layer of AlGaN.
Japanese patent application laid-open No. 2000-357663 (prior art 2) discloses an ELO crystal growth method that is based on formation of semiconductor film (=first semiconductor film) with steps.
“Transmission Electron Microscopy Investigation of Dislocations in GaN Layer Grown by Facet-Controlled Epitaxial Lateral Overgrowth”, JAPANESE JOURNAL OF APPLIED PHYSICS, Vol. 40 (2001), pp. L309–L312, Part 2, No. 4A, 1 Apr. 2001 (prior art 3) discloses a crystal growth method for reducing the number of dislocations. Prior art 3 also describes about the crystal growth process that voids or dislocations are formed.
Hiromitsu Mizutani et al., “Reduction of dislocation density in GaN using FACELO (Facet Controlled Epitaxial Lateral Overgrowth)”, TECHNICAL REPORT OF IEICE, ED2000-22, CPM2000-7, SDM2000-22(2000–05)(prior art 4) discloses a crystal growth method that formation of void can be controlled by adjusting a ratio of a width of region to be covered with ELO mask (mask width) and a width of region not to be covered with ELO mask (window width) on the surface of a crystal growth substrate or by adjusting the conditions of crystal growth (temperature, flow rate of source gas etc.). However, prior art 4 is based on intentional formation of voids that the number of dislocations is reduced by terminating the growth of dislocation at a void site.
Problems caused by the formation of void will be described below.
FIG. 1 is a schematic cross sectional view showing an ELO mask 3 and its vicinity to illustrate the formation of void in prior art method. The ELO mask 3 of amorphous material is formed on a crystal growth surface 1a of crystal growth substrate 1. When a semiconductor crystal 2 of e.g., GaN is grown on the crystal growth surface 1a with the ELO mask 3, voids 4 or dislocations are generated.
Prior art 3 describes an example of formation of voids of about 3 to 8 μm. In such a region with voids formed, it is impossible to form a continuous and monolithic semiconductor layer (e.g., n-type contact layer). So, in order to form a semiconductor device structure (e.g., semiconductor layer after n-contact layer) to be further formed thereon, the crystal growth has to be continued until when having a continuous flat plane of crystal growth surface while covering the voids. Thus, when a semiconductor device structure is formed on a semiconductor crystal with voids, a thick layer of semiconductor crystal must be grown such that it reaches above the height of voids. Therefore, the productivity must lower.
Further, when a thick layer of semiconductor crystal is thus formed, a deformation such as warp occurs around the interface due to a difference in thermal expansion coefficient between a crystal growth substrate and a semiconductor crystal layer formed thereon. There may occur a problem in subsequent alignment step.
Further, in aspect of external quantum efficiency, it is undesirable to have such a void since part of light emitted from a light emitting element formed on the semiconductor crystal 2 may be diffused by such a void.
As described, prior arts 1 and 2 relate to some crystal growth method to reduce the occurrence of void to make a good semiconductor crystal.
On the other hand, crystal growth methods, called stepwise ELO, to reduce the number of dislocations in semiconductor crystal without using any ELO mask are known (Japanese patent application laid-open Nos. 2002-164296 (prior art 5) and 2002-280609 (prior art 6), “High Output Power InGaN Ultraviolet Light-Emitting Diodes Fabricated on Patterned Substrates Using Metalorganic Vapor Phase Epitaxy”,
JAPANESE JOURNAL OF APPLIED PHYSICS, Vol. 40(2001), pp. L583–L585, Part 2, No. 6B, 15 Jun. 2001 (prior art 7), and “InGaN-Based Near-Ultraviolet and Blue-Light-Emitting Diodes with High External Quantum Efficiency Using a Patterned Sapphire Substrate and a Mesh Electrode”, JAPANESE JOURNAL OF APPLIED PHYSICS, Vol. 41(2002), pp. L1431–L1433, Part 2, No. 12B, 15 Dec. 2002) (prior art 8).
The above crystal growth methods are conducted such that an effective uneven pattern to cause a lateral growth of semiconductor crystal is provided on the crystal growth surface of a crystal growth substrate. Thus, the lateral growth of semiconductor crystal is used to have a good semiconductor crystal.
However, in prior arts 1 and 2, there are some requirements in the process that the AlGaN layer is to be formed as an underlying layer and that the semiconductor film with steps is to be formed. These requirements may further complicate the semiconductor crystal growth process for light emitting element that is complicated recently. Namely, in case of using these methods, it is difficult to obtain a good semiconductor crystal while simplifying the crystal growth process.
In the crystal growth method called stepwise ELO as disclosed in prior arts 5 to 8, it is impossible to arbitrarily select or adjust the refractive index of convex portion independently of that of a material of crystal growth substrate since the convex portion on crystal growth surface is made of the same material as the crystal growth substrate. Namely, in the above stepwise ELO, it is theoretically impossible to select or adjust, as in case of using an ELO mask, the material of convex portion on crystal growth surface to enhance the external quantum efficiency.